This invention relates to the selective preparation of organic trisulfides. More particularly, it relates to a process for producing a substantially pure organic trisulfide from mercaptan and a high rank polysulfide as a sulfur source wherein the high rank polysulfide is reacted with mercaptan in the presence of a heterogeneous alumina-based catalyst, alkali metal or alkaline earth metal modified silica-based catalyst, zinc-based catalyst, or mixtures thereof. The process is operated either as a single step batch process, a continuous single-reactor process, or as a final step of a continuous multiple-step process. According to one embodiment the high rank polysulfide is earlier prepared by reacting a mercaptan of the formula RSH with elemental sulfur, which process may be characterized by the overall reaction illustrated by the following equation (1):
2 RSH + 2S Catalyst RSSSR + H₂S (1)
While the above equation demonstrates the stoichiometry of the reaction, surprisingly, in practice, an excess of mercaptan (RSH) is much preferred for efficient production of the pure trisulfide.
Organic polysulfides, for example, t-butyl polysulfides, t-nonyl polysulfides and the like, have excellent extreme-pressure (E.P.), anti-wear and anti-weld properties and are widely used as E.P. additives in lubricant compositions such as metal-working fluids and high pressure gear lubricants. However, polysulfides (RSSxSR, where x is greater than or equal to 2) exhibit high copper-strip corrosivity (ASTM Copper Strip Corrosion Test D-130-56), rendering them unsatisfactory for many commercial lubricating oil applications such as automotive and industrial gear oils. Organic trisulfides, on the other hand, have both optimum sulfur assay and low copper-strip corrosivity making them the ideal E.P. lubricant additives for applications where corrosivity to metals can not be tolerated.
The reaction of mercaptans with sulfur in the presence of certain basic catalysts is known for the production of a mixture of polysulfides of varying sulfur rank. US-A-4 564 709 addresses the selective synthesis of dialkyl trisulfides by reacting sulfur and mercaptan over a magnesium oxide catalyst. The product contains significantly large amounts of tetrasulfide in comparison to the present invention and, in laboratory test operation, the magnesium oxide lost substantially all activity after one batch of material was produced.
US-A-3 392 201 teaches the production of a product high in alkyl trisulfide by treating a material which contains alkyl trisulfides and alkyl polysulfides (more than 3 sulfur atoms per molecule) with at least one alkali metal hydroxide or ammonium hydroxide whereby the polysulfides are converted to trisulfides.
Vineyard, J. of Org. Chem 32, 3833-6 (1967) and GB-A-1 162 334 to Monsanto disclose that an excess of mercaptan is required to react with sulfur for the synthesis of various polysulfides using a liquid amine catalyst. The reaction conditions require the presence of a polar solvent (e.g, methanol) and the trisulfide produced is of low purity.
The above-identified prior art has also failed to recognize the importance of the mercaptan (mole) to sulfur (g-atom) ratio. The preferred ratio range taught in US-A-4 564 709 is 1.25 to 1.7 while the broad range given is 1.0 to 2.0.
Vineyard teaches that an excess of mercaptan is needed but demonstrates in his working example, that a mercaptan (mole) to sulfur (g-atom) ratio of 1.25 is sufficient.
EP-A-0 506 347, which is a document within the meaning of Article 54(3) EPC, relates to the production of dihydrocarbyl polysulfides: hydrocarbyl mercaptan is reacted with sulfur in the presence of added alumina catalyst. With tertiary alkyl mercaptans high yields of di-tert-alkyl trisulfides are formed with excellent selectivity. With primary alkyl mercaptans the process can be controlled to form products enriched in either dialkyl disulfides or dialkyl trisulfides, and in either case little or no tetrasulfides are formed. The process is environmentally friendly in that halogen-containing reactants such as sulfur chlorides are not used and thus the products are not contaminated with significant amounts of halogen residues.
EP-A-0 202 420 relates to a continuous process for the manufacture of dialkyl disulfides by reacting an alkyl alcohol and hydrogen sulfide in one reaction zone, and then passing the reactor effluent into a second reaction zone where it is reacted with elemental sulfur in the presence of a solid, particulate catalyst.
EP-A-0 342 454 discloses a process for continuously preparing dialkyl disulfides and dialkyl polysulfides by reacting at elevated temperature and in the presence of a solid, particulate catalyst an alkene and hydrogen sulfide in a first reaction zone, and then passing the reactor effluent into a second reaction zone where it is reacted at elevated temperature with molten, elemental sulfur in the presence of a solid, particulate catalyst.
Journal of the society of Chemical Industry 67(1), pp. 14-17 (1948) is a paper on the reaction of sulfur and sulfur compounds with olefinic substances. The disclosure of Journal of the society of Chemical Industry 67(1), pp. 14-17 (1948) relates in particular to rubber vulcanization. The teaching of this document, however, is restricted to the discovery that sulfur can be reacted with mercaptans in the presence of zinc oxide to produce di- and polysulfides. In its experimental part Journal of the society of Chemical Industry 67(1), pp. 14-17 (1948) describes the reaction of sulfur and zinc oxide with several mercaptans. However, this document does not report the production of any trisulfides.
This invention is a process for the selective production of organic trisulfides of the formula R¹SSSR² where R¹ and R² are independently C₁ to C₂₄ monovalent hydrocarbon radicals optionally having hydroxy or alkoxy substituent groups which process comprises reacting in the absence of a polar solvent a high sulfur rank polysulfide as the major sulfur source of the reaction, said polysulfide having the formula R¹SSx-1SR² where x is greater than 2 and R¹ and R² are as defined above, with a mercaptan of the structure R³SH, where R³ is of the same description as R¹ and R², the ratio of mercaptan/polysulfide ranging from 1.0 to 10.0, at a reaction temperature ranging from 30 to 150°C and in the presence of a heterogeneous catalyst which is a) an alumina-based catalyst, b) a silica-based catalyst containing at least 0.5 weight percent of either sodium, potassium, calcium or magnesium, or c) a zinc-based catalyst and terminating the reaction in sufficient time to permit recovery of a total product containing at least 70 weight percent of organic trisulfide of the above formula, at least a portion of which is the product of the reaction of said high sulfur rank polysulfide and said mercaptan.
In the drawing, Figure 1 is a flow diagram for a continuous multi-step embodiment of this invention. Figure 2 shows a series of distillation towers which may be used for purification of crude mercaptan produced in Zone 1 of Figure 1, and Figure 3 is a second series of distillation towers for the removal of H₂S, and mercaptan from pure organic trisulfide produced in Zone 3 of Figure 1.
In more detail, this invention is a method for selectively producing an organic trisulfide of the formula R¹SSSR² where R¹ and R² are independently C₁-C₂₄ substituted or unsubstituted monovalent hydrocarbon radicals preferably including alkyl, aryl, hydroxy, alkoxy or amino substituted alkyl or aryl groups, or alkaryl groups. More preferably, R¹ and R² are alkyl radicals having 1 to 12 carbon atoms. For example, the alkyl radicals are methyl, ethyl, propyl, butyl, hexyl, octyl, nonyl, decyl and dodecyl groups. The trisulfides are produced by reacting one or more mercaptans having the structure R³SH, where R³ is the same as R¹ and R², with sulfur or a polysulfide of the formula R¹SSx-1SR², where x is greater than 2, in the presence of a heterogeneous (insoluble in the reaction mixture) alumina-based catalyst, alkali metal or alkaline earth metal modified silica-based catalyst, zinc-based catalyst, or mixtures thereof, and recovering a product which is at least a major proportion of trisulfide relative to the total weight of other sulfides in the product. At least about 70 % by weight of the product is trisulfide.
The letter x in the above formula represents the average number of internal sulfurs in a given organopolysulfide composition and not the maximum number of sulfur atoms for any one species of polysulfide in the composition. The term "high rank" polysulfide refers to compounds or mixtures of compounds wherein the polysulfide contains labile sulfur atoms and where x in the above formula is greater than 2.
The selective production of pure trisulfides, as opposed to a product having a distribution of disulfide, trisulfide, tetrasulfide, etc., is not overly discussed in the relevant prior art, patent references GB-A-1 162 334, US-A-4 564 709 and US-A-3 339 201 being exceptions. The present invention not only provides a method for selectively producing organotrisulfides but also furnishes the capability of yielding material of a selected distribution of disulfide, trisulfide and tetrasulfide. A selective distribution of di-, tri- and tetra- polysulfides can be produced by controlling the process conditions. High disulfide levels are favored by higher temperature, increased mercaptan to sulfur ratios, and longer residence times in Zone 2 while temperature and residence time control the amount of disulfide produced in Zone 3 of the accompanying drawing. The level of tetrasulfide desired is controlled by selecting the proper mercaptan to sulfur ratio in Zone 3. Higher values for this ratio produce lower levels of tetrasulfide. For example, if one wanted a distribution containing a high disulfide level and a high tetrasulfide level then the conditions employed in Zone 3 would be high temperature and low mercaptan to sulfur ratio. However, if low disulfide and low tetrasulfide levels are desired then a low temperature and a high mercaptan to sulfur ratio would be employed.
As previously stated, the overall reaction of this process is illustrated by equation (1)
2 RSH + 2S Catalyst RSSSR + H₂S (1)
When the progress of the reaction is followed as a function of time, it is observed that two parallel reactions take place; a fast reaction, as shown in equation (2), produces mostly high rank (x>2) polysulfides and the largest portion of H₂S
and a slow reaction, as shown in equation (3), which consumes the high rank (x>2) polysulfides of equation (2), producing predominantly trisulfide.
If the reaction is not stopped upon completion of the reaction of equation (3), then the trisulfide (RSSSR) will be converted to the disulfide (RSSR) as shown in equation (4)
RSSSR + 2RSH Catalyst 2RSSR + H₂S (4)
Accordingly, the reaction of equation (3) is carried out for a time sufficient to produce trisulfides and terminated before the formation of appreciable quantities of disulfides as in equation (4). The period of time for the production of a predominent amount of trisulfides and before the formation of appreciable amounts of disulfide will vary greatly depending on the reactor design and reaction conditions employed. However, under each set of conditions the time element may be readily determined empirically by periodic sampling and analysis of the product.
An important aspect of this invention, if very pure trisulfide is desired, is the disengagement of H₂S (hydrogen sulfide) produced from the reaction of mercaptan with sulfur. This is required to force the reaction, as illustrated by equation (3), to completion. The removal of H₂S from the reactor may be accomplished, for example, by purging the reaction mixture with an inert gas, cycling the reaction mixture through a pressure drop, spraying the mixture on a heated surface with a large surface area, or by refluxing the reaction mixture.
As previously mentioned, the process of the invention can either be operated as a single step, batch process, a continuous, single reactor process or as the final step of a continuous, multi-step process with separate reaction conditions in each reaction zone.
The catalyst used for the trisulfide producing reaction in any of these systems is a heterogeneous alumina-based catalyst, alkali metal or alkaline earth metal modified silica-based catalyst, zinc-based catalyst or mixtures thereof. Alumina-based catalysts are those including, for example, alumina, titania-silica-alumina, titania-alumina, and silica-alumina.
Silica-based catalysts contain at least 0.5 weight percent of sodium, potassium, calcium or magnesium. Silica by itself shows relatively slight activity, however, silicas containing a basic metal content of at least 0.5% by weight are very good catalysts in this process. Alkali metals, particularly sodium, are the preferred basic metals for silica-based catalysts.
Zinc-based catalyst are zinc oxide or support catalysts containing zinc; for example, zinc-alumina, silicas containing zinc, and titania containing zinc.
All of the catalysts referred to above are effective for the process described herein but the preferred are alumina, zinc-alumina, zinc-silica-alumina and titania-silica-alumina. The activity of various catalysts differs mainly in the ability of a selected catalyst to produce a trisulfide with both a very low tetrasulfide content (less than 1 wt.%) and a low disulfide content (less than 5 wt.%).
The ratio of mercaptan (moles) to sulfur [gram-atom (g-atom)] can range from 1.0 to 10.0 depending on the purity of product desired. For example, in the case of t-butyl trisulfide, if a very pure product is desired, then the preferred range lies between 3 and 10 and more preferably between 5 and 10. In general, the preferred range is between 3 and 10. Material produced within this range has the lowest residual tetrasulfide content. When a high rank polysulfide is used as the sulfur source, the amount of mercaptan used is adjusted to the amount of sulfur generated by the polysulfide to fit within the above-described mercaptan (moles) to sulfur (g-atom) ratio range.
The prior art suggests that a very high mercaptan to sulfur ratio is undesirable because large amounts of disulfide would be produced under these conditions. This would be true if the reaction were not stopped in time as described above, However, the mercaptan to sulfur ratio controls the point where the minimum level of high rank (x>2) polysulfide is achieved before significant amounts of organic disulfide are produced. For example, if t-butyl mercaptan is reacted with sulfur at a molar ratio of 1.7, the minimum level of t-butyl tetrasulfide produced before significant quantities of t-butyl disulfide are generated, is about 5%. However, if under the same conditions a mercaptan to sulfur molar ratio of 5 is employed, then the minimum level of t-butyl tetrasulfide achievable is less than 1% before significant quantities of t-butyl disulfide are generated. This realization is completely unobvious in view of the disclosure of the prior art relative to mercaptan to sulfur ratio.
The temperature range of the process is generally between about 30 and 150°C, preferably between about 50 and 100°C. When operating the process in a batch or continuous single reactor, the range is from 35 to 150°C. Of course, the temperature in these systems is limited by the pressure limitation of the reaction vessel and the reflux temperature of the reaction medium. The most preferred method of operation, independent of the type of reactor used, is at reflux temperature of the reaction medium, provided the reactants and products are stable at this temperature. If a volatile mercaptan is employed, the reaction can be operated first at a lower temperature, where the initial stages of the reaction occurs and the largest majority hydrogen sulfide is evolved [equation (2)], and then at higher temperature within the prescribed range to complete the conversion of high rank (x>2) polysulfides to trisulfide [equation (3)]. This will reduce the loss of the volatile mercaptan. The temperature in the reaction zone in a continuous-multiple reactor process, where the catalyst bed comprises an alumina-based catalyst, ranges from about 30 to about 150°C, preferably between 45 and 120°C but most preferably between 60 and 100°C.
In any of the reactor processes, the reaction pressure in the mercaptan-sulfur reaction zone ranges from ambient to about 2.17 MPa (300 psig), preferably from ambient to 1.8 MPa (250 psig).A lower pressure, within this range, is preferred depending on the volatility of the mercaptan. Operation of the process at a pressure at which the H₂S evolved is dissolved in the liquid phase is advantageous.
In the process of this invention, a polar solvent such as methanol, as required in the process of Vineyard, supra, is not needed to produce pure trisulfide product. In addition, catalyst deactivation is not observed as for the magnesium oxide catalyst of US-A-4 564 709.
The type of reactor chosen for operation of the process in a continuous, single step depends on the reaction conditions desired and whether sulfur or a high rank (x>2) polysulfide is used as the sulfur source. If sulfur is used, and the reaction temperature is below the melting point of sulfur, then a continuous-stirred-tank-reactor is required. If a high rank (x>2) polysulfide is employed, then a fixed-bed reactor can be utilized.
When the use of a continuous, multiple-step reactor process is contemplated, organic trisulfides are produced from a starting alcohol or olefin reacted with H₂S and sulfur, the resultant high rank (x>2) polysulfide is then converted to the trisulfide by a controlled reaction with a mercaptan. The alcohol or olefin and H₂S are reacted in a first reaction zone to form crude mercaptan; the mercaptan-containing reactor effluent is stripped of H₂S (and water removed, if present) and then passed directly into a second reaction zone where it is reacted with sulfur to produce an organic polysulfide of high sulfur rank (x>2). Depending on the purity of the product desired and the conversion of the mercaptan being produced, the mercaptan may or may not require further purification. The intermediate polysulfide is then reacted with additional mercaptan to produce a pure organic trisulfide [equation (3)] in a third reaction zone, the reaction with mercaptan generally terminating with the formation of the trisulfide and before any formation of substantial amounts of disulfide.
Further details of the continuous, multiple step process are seen in the accompanying drawing wherein Figure 1 is a flow diagram of the continuous, multiple-step process. In Zone 1 of Figure 1, an olefin or an alcohol is fed continuously through preheater 2 and H₂S is fed continuously through preheater 4 to a first reactor 6 to produce a crude mercaptan from which water (if initial reactant is an alcohol) is removed in liquid-phase water separator 8. The crude mercaptan is then passed to a high-pressure separator 10 from which a major portion of the residual H₂S is removed overhead and recycled back to preheater 4. The effluent removed from the bottom of separator 10, consisting of crude mercaptan and residual H₂S, is passed through preheater 12 into reactor 14 of Zone 2. Sulfur is charged to reactor 14 via preheater 16 and the formed crude high rank polysulfide is passed from the bottom of reactor 14 to distillation column 18 where H₂S is removed overhead and recycled to preheater 4. The bottoms stream from distillation column 18 is passed to reactor 20 of Zone 3 containing a catalyst of this invention. Any additional mercaptan required to adjust the overall mercaptan (mole) to sulfur (g-atom) ratio, as prescribed, is also supplied at the bottom of reactor 20. In reactor 20, the high rank (x>2) polysulfides are converted to pure trisulfide in accordance with this invention and as represented by equation (3), and then discharged overhead from reactor 20. The preferred equipment for Zone 3 is a fixed-bed, up-flow-reactor. However, other equipment can be used including, for example, fixed-bed, down-flow; continuous-stirred-tank (CSTR); external transport; or other reactor designs similar to those used in reactive distillation processes.
The reactions of Zones 1 and 2 are more fully described in US-A-5 026 915, and US-A-4 937 385.
In Figure 2, a system is shown which may be used to further purify the crude mercaptan effluent from high-pressure separator 10, if necessary. Crude mercaptan from separator 10 is directed to distillation tower 22 of Figure 2 where the mercaptan and excess alcohol or olefin (overhead distillate) are separated from organosulfide (bottoms). The olefin or alcohol (overhead) is then separated from the mercaptan (bottoms) in distillation tower 24. The olefin or alcohol is recycled to reactor 6 via preheater 2 of Zone 1 and the mercaptan is passed to reactor 14 of Zone 2.
To separate the pure trisulfide product of reactor 20 (Zone 3) from H₂S and mercaptan, the overhead from reactor 20 may be passed to distillation tower 26 of Figure 3 where H₂S is removed overhead and recycled to reactor 6 via preheater 4. The bottoms product of tower 26 is then sent to distillation tower 28 where the majority of mercaptan is removed overhead and recycled to reactor 14. The bottoms product of tower 28 is then passed to vacuum distillation tower 30 for the overhead removal of the last traces of residual mercaptan which also may be recycled, if desired. Pure trisulfide is recovered from the bottom of tower 30.
While the flow diagram of Figure 1 describes a continuous process, a batch process could also be utilized in Zone 1, Zone 2 and/or Zone 3, if desired.
The following examples are set forth to demonstrate the operation of the process of this invention.
The apparatus used for this work consisted of a 100 ml, 3-necked round bottom flask equipped with a temperature controlled heating mantle and magnetic stirrer. Fitted to the flask were a thermocouple well, a septum inlet with stopcock, and a glycol cooled (10°C) condenser. The vent from the condenser was connected to a burner line for odor control.
The reactor flask was charged with the desired catalyst and sulfur at a ratio (by weight) of approximately 3.3 and sealed. Via the septum inlet, t-butyl mercaptan was then added and the mixture was heated to reflux with stirring. Samples were taken at various intervals and analyzed by gas chromatography. Table 1 shows the results for a variety of catalysts and conditions used in runs 1-17. All runs were carried out at atmospheric pressure.
Runs 1-8 show the effectiveness of a variety of alumina-based catalysts.
Run 9 shows the effectiveness of silica with sodium content greater than 0.5% as a catalyst.
Runs 10-11 show the effectiveness of zinc-based catalysts.
Runs 12-15 show the effect of the mercaptan (mole) to sulfur (g-atom) ratio on the purity of the trisulfide.
Runs 16-17 show the effect of the removal of hydrogen sulfide by reflux on the reaction. Run 17 was conducted at 54°C which is below the reflux temperature (65°C) of the reaction mixture. All other runs were conducted at the reflux temperature. After 40 minutes, the minimum level of tetrasulfide obtained in run 17 was 1.5% versus 0.5% obtained after 30 minutes at reflux (Run 16).
In comparison to the foregoing results, several experiments were performed to demonstrate relevant prior art processes.
This example illustrates the inability of amine catalysts of the prior art to produce high purity trisulfides from mercaptans and sulfur.
The apparatus used for this work consisted of the same equipment used in Example 1.
The reactor flask was charged with 1.23g of sulfur. Via the septum inlet, 16.9g of t-butyl mercaptan, 4.76 ml of methanol, and 0.075 ml of n-butyl amine were then added and the mixture was heated to reflux with stirring for 4 hours. Analysis by gas chromatography, showed the levels of tert-butyl disulfide, tert-butyl trisulfide, and tert-butyl tetrasulfide to be 0.28%, 88.84%, and 10.86%, respectively, on a mercaptan-free basis.
This example shows that magnesium oxide is an effective catalyst for producing pure trisulfides (Prior Art); however, this catalyst loses activity after one run.
The apparatus used for this work consisted of the same equipment used in Example 1.
The reactor flask was charged with 1.9971 g of sulfur and 0.2535 g of magnesium oxide and sealed. Via the septum inlet, 28g of t-butyl mercaptan were then added and the mixture was heated to reflux and stirred for 5 hours. The reaction mixture was sampled after 1.5 hours and analysis by gas chromatography, showed the levels of tert-butyl disulfide, tert-butyl trisulfide, and tert-butyl tetrasulfide to be 1.12%, 97.6%, and 1.27%, respectively, on a mercaptan-free basis. Analysis of reaction mixture after 5 hours by gas chromatography, showed the levels of tert-butyl disulfide, tert-butyl trisulfide, and tert-butyl tetrasulfide to be 2.5%, 96.5% and 0.89%, respectively, on a mercaptan-free basis.
The mixture was allowed to cool and settle overnight. The liquid was carefully removed and the procedure was repeated using the same magnesium oxide. No reaction was observed.
Runs 18-23 of Table 1A demonstrate that the class of catalysts of this invention do not lose activity after one run as was the case with magnesium oxide. The same charge of catalyst was used successively for Runs 18-23 by carefully removing the reaction mixture at the end of each run after the catalyst had been given sufficient time to settle. The apparatus used was the same equipment used in Example 1. The catalyst was Harshaw 377A-2-1-16, a zinc-alumina, and the t-butyl mercaptan to sulfur ratio was 5 in each run.
The apparatus used for this work consisted of the same equipment used in Example 1.
The reactor flask was charged with the 4.0g of the desired catalyst and sealed. Via the septum inlet, 5.9g of t-butyl mercaptan and 3.2g of t-butyl polysulfide¹ were added and the mixture was heated to reflux with stirring. Samples were taken at various intervals and analyzed by gas chromatography. The results are presented in Table 2.
¹ The t-butyl polysulfide used as feed stock for this Example contained 5.2% t-butyl disulfide, 60.2% t-butyl trisulfide and 34.6% t-butyl tetrasulfide based on gas chromatography analysis.
Tert-butyl mercaptan, sulfur, and hydrogen sulfide were passed through a 3.8 cm (1.5 inch) fixed-bed reactor containing 100g UOP's LX-Y52 zeolite at molar velocities of 1,000, 700, and 200, respectively. The temperature of the reactor was 130°C and the pressure was 2.5 Mpa (350 psig). The reactor effluent was collected in a stainless-steel cylinder at 25°C and 0.8 MPa (100 psig). Analysis by gas chromatography, showed the levels of tert-butyl disulfide, tert-butyl trisulfide, and tert-butyl tetrasulfide to be 1.4%, 54.5%, and 44.1%, respectively, on a mercaptan-free basis.
The collected reactor effluent (di-tert-butyl polysulfide and tert-butyl mercaptan) and additional tert-butyl mercaptan were then passed through a one-inch fixed-bed reactor containing 57.7g of Harshaw's Zinc-Alumina (377A) in an up-flow direction at 0.45 g/min and 0.6 g/min, respectively. The temperature of the reactor was at 60°C and the pressure was ambient. Analysis of the reactor effluent by gas chromatography, showed the levels of tert-butyl disulfide, tert-butyl trisulfide, and tert-butyl tetrasulfide to be 3%, 93.7%, and 2.1% respectively, on a mercaptan-free basis.
Similar excellent results for the preparation of a product containing almost pure t-nonyl trisulfide or t-dodecyl trisulfide can be obtained using the same equipment and procedure as described in Example 3 with either a t-nonyl mercaptan or t-dodecyl mercaptan starting reactant in place of t-butyl mercaptan.